1. Field of the Invention
This invention relates generally to the chemical vapor deposition of material onto a substrate and, more particularly, to the use of an axially symmetric gas flow for improving the deposition onto a substrate of a material carried by the gas.
2. Discussion of The Related Art
It is known in the related art of chemical deposition of a material onto a substrate, to provide a susceptor in an enclosed container, the susceptor typically supporting a plurality of substrates. A carrier gas, containing gaseous forms of the atoms to be deposited on the substrate, is introduced into the container in the vicinity of the susceptor. The flow of the gas, determined by the geometry of the container and the susceptor, is generally constrained to flow parallel to the substrates. By a combination of transport and chemical reaction, the atoms of the deposition materials adhere at high temperature to the substrate surfaces, forming the desired deposition layer. This deposition technique has proven satisfactory in the part, however as higher volumes of material have been required and higher quality materials needed, limits of this technique are being reached. The deposition technique has four problems. The first problem is that, as the gas flows over the surfaces of the substrates and the susceptor, deposition of the material onto the surface changes the concentration of the deposition materials in the carrier gas. Consequently, over the length of the susceptor, and indeed over the length of each substrate, a different rate of growth of the layer of material is found. A second problem is that, as the deposition material is depleted in the region of deposition, new deposition material must be transported across relatively long distances in the large reaction containers used for deposit. This transport controlled deposition limits the rate at which deposition can occur, and therefore, is related to the cost of the manufacture of the materials such as in the epitaxial process. A third problem is generally referred to as autodoping. In the autodoping process, impurity atoms from the highly doped substrate can be detached from the substrate surface and can be incorporated via the gas phase into the more lightly doped layer of material being deposited. In the related art, special steps must be taken to minimize autodoping, such as deposition of an extra coating onto the back of the substrate. A final problem is particulate contamination. As chemical vapor deposition chambers have become larger, the wall area of the chamber has increased. Unwanted deposits that form upon these walls are sources for particulates that can be inadvertently incorporated into the deposition material.
A need has therefore been felt for a technique of vapor deposition in which the growth rate of the deposited material onto a substrate is highly uniform over the entire area of the substrate, in which the growth rate of the deposited material can be increased, in which autodoping of the deposited material can be prevented without additional process steps, and in which particulate contamination can be minimized.